STRENGTHENING OF CONCRETE CONSTRUCTIONS WITH EXTERNALLY BONDED REINFORCEMENT:

DESIGN CONCEPTS AND CASE STUDIES

S. Ignoul1 K. Brosens2 D. Van Gemert3

ABSTRACT The paper deals with the design philosophy of externally bonded reinforcement. The difference between strength requirements (ultimate limit state) and deformation requirements (serviceability limit state) and its consequences on the use of either steel or FRP will be clarified. Strengthening can be required in bending as well as in shear. The use of either steel or CFRP depends on the specific requirements of additional bending or shear reinforcement, including deformability and toughness aspects. It is also possible to combine metallic and non-metallic elements. In that way a synergy between the specific properties and benefits of both materials can be obtained. Another important issue is the correct design of the anchorage zone, where the force transfer between the concrete structure and the external reinforcement will take place. A practical example will illustrate the application of polymer adhesives and strengthening elements RESUME La philosophie du dimensionnement des armatures extérieures est traitée dans cette contribution. La différence entre les critères de résistance (ELU, état limite ultime) et les critères spécifiés d’exploitation, comme des déformations excessives (ELS, état limite de service) et sa conséquence pour le choix des différents matériaux est clarifiée. Les renforcements sont dimensionnés pour reprendre des moments fléchissants ou des efforts tranchants. Le choix de l’usage de l’acier ou des tissus de fibres carbones (TFC) dépends des contraintes du projet (moment fléchissant ou effort tranchant, déformations,…). C’est bien possible de combiner des éléments métalliques et non-métalliques. Ainsi les caractéristiques spécifiques et les bénéfices des deux matériaux sont combinés. Un aspect très important est le dimensionnement de la longueur d’ancrage. C’est la longueur minimale qui est nécessaire pour transférer les forces entre l’armature extérieure et le béton. Enfin, un cas pratique des renforcements extérieurs est ajouté. Key words: Strengthening, externally bonded reinforcement, CFRP, design, anchorage 1. S. Ignoul Corresponding author, Triconsult N.V., Industriepark 1241/4,

3545 Halen, Belgium sven.ignoul@triconsult.be

2. K. Brosens Triconsult N.V., Industriepark 1241/4, 3545 Halen, Belgium kris .brosens@triconsult.be

3. D. Van Gemert K.U.Leuven, Kasteelpark Arenberg 40, 3001 Heverlee, Belgium dionys.vangemert@bwk.kuleuven.ac.be

INTRODUCTION

The strengthening of concrete structures with externally bonded reinforcement is generally done by using either steel plates or CFRP laminates. Each material has its specific advantages and disadvantages. Steel plates have been used for many years due to their simplicity in handling and applying and to their effectiveness for strengthening. The properties and behaviour of steel-concrete structures are well known. Steel plates are very effective to be used as bending reinforcement. The high tensile strength and stiffness lead to an increase in bending capacity and a reduction of the deformations. Steel plates can also be used as external shear reinforcement. However, labour costs might rise quickly. Steel stirru ps have to be bend or welded and very often anchored with bolts in the concrete compression zone. When several stirrups per meter are needed, these costs can make this technique economically less interesting. CFRP sheets have very high tensile strength and stiffness, figure 1. Nevertheless, they cannot be used in every strengthening situation. When used in bending, the active stresses in the CFRP laminates have to be kept small in order to prevent the internal steel reinforcement from yielding. This means that the high strength properties of the CFRP sheets are not used effectively. The required increase of bearing capacity can only be reached by adding a considerable number of sheets, which increases the material and labour costs. For limiting the deflections, CFRP sheets are not very effective. Due to their very small cross sectional area per sheet, the moment of inertia will only slightly increase and so the deformation decrease will only be marginal. In these cases, very often steel plates offer a better alternative. On the other hand, CFRP sheets are more appropriate for shear strengthening than steel plates. An orthogonal net of carbon fibres bonded at both sides of a beam is very well able to take shear forces. The applying of the CFRP sheets is very easy. Even complex shapes and geometries can be done. Labour costs are considerably lower for CFRP than for externally bonded steel stirrups.

Steel

CFRP

F

,

Fsy

,s y

FCFRPu

,s u,CFRPu

Figure 1. Stress-strain relationship of CFRP in comparison with steel

To combine the features of the two materials, a hybrid steel/CFRP strengthening method can be developed. The additional longitudinal reinforcement consists of externally bonded steel plates, whereas the shear reinforcement consists of externally bonded CFRP sheets.

The behaviour of such hybrid strengthening was extensively studied in an experimental program set up at the Reyntjens Laboratory, K.U.Leuven, Belgium, see Brosens et al1. Even when the external CFRP shear reinforcement is applied only at one side of the beam, a considerable increase of the shear capacity could be obtained.

BENDING DESIGN

For the design of the externally bonded reinforcement, all material properties and geometrical characteristics of the cross section of the existing concrete element must be known, or have to be determined experimentally. Furthermore, the magnitude of the load acting on the element at the moment of gluing the external reinforcement and the new load acting on the strengthened cross section must be known. The stress distribution at the moment of gluing the external plates is determined by the moment M0 , which is called the unloading moment. This unloading moment can be changed by removing part of the permanent loads, or by complete ly or partly jacking up the element. The more a structure is unloaded, the more effective the additional external reinforcement will be, which allows a smaller cross sectional area of the external reinforcement, see Van Gemert et al2. The design method gives the cross sectional area of the external reinforcement. When steel plates are used, it concerns the thickness and the width of the plate, while with CFRP laminates, it concerns the number and the width of the CFRP sheets. According to the European Standard ENV 1992-1-1, see Eurocode 23, two limit states have to be checked: the ultimate limit state and the serviceability limit state. The ultimate limit state is the state where the concrete structure completely fails or collapses. In the calculations, the design values of the loads and the material properties are used. Non-linear stress-strain relationships for steel and concrete are used allowing plastic deformations. However, the CFRP reinforcement can be considered as a perfectly linear elastic material. Once the tensile strength is reached, a brittle fracture occurs suddenly. The assumption is made that the strains vary linearly over the height of the beam. The ultimate limit state assumes that at least one of the materials attains its maximum allowable strain. The cross sectional area of the external reinforcement and the position of the neutral axis is determined by using the equilibrium of normal and bending forces over the concrete section. The serviceability limit state is the state beyond which specified service requirements are no longer met. These requirements concern limited deformations or deflections and low vibrations. In the serviceability limit state, plastic deformations are not allowed. Linear stress-strain relationships and nominal values of the loads are used to prevent large deflections and wide crack openings. Excessive deflections are not necessarily dangerous, but people will not accept them for aesthetical and psychological reasons. Due to large cracks in the concrete cover, the internal steel bars could be exposed to aggressive environmental atmospheres causing steel corrosion. Therefore wide crack openings must be avoided. This can be realised by limiting the stresses under service conditions to a fraction of the limit of elasticity. Eurocode 2 limits the concrete compressive strength to 50% or 60% of the characteristic cylinder compressive strength depending on the environmental circumstances whereas the steel stress is limited to 80% of the characteristic value of the yield strength. The maximal value of the deflections is often imposed by the customer or can be found in design codes.

SHEAR DESIGN

When a concrete element has insufficient internal shear reinforcement, additional shear reinforcement can be bonded to the sides of the element. This additional shear reinforcement can consist of either externally bonded steel stirrups or externally bonded CFRP sheets. The design is done in the ultimate limit state according to Eurocode 23 . For the design, it makes no difference whether steel stirrups or CFRP sheets are used for shear strengthening. Since CFRP laminates are able to transfer forces only in the direction parallel to the fibres, the best solution would theoretically consist in applying externally bonded CFRP shear reinforcement perpendicular to the shear cracks. Generally, these shear cracks form an angle of 45° with the longitudinal axis of the beam. In practice, however, it is nearly unfeasible to apply CFRP laminates with an angle of 45°. Therefore an orthogonal web of carbon fibres has to be used, since the shear transfer between individual carbon fibres is very low. One half of the fibres is oriented horizontally, whereas the other half is oriented vertically, which means that the total number of layers is a multiple of two, figure 2.

Shear crack (45°)

Orthogonal web ofcarbon fibres(horizontal & vertical)

Shear crack (45°)

Carbon fibres with an angle of 45°

Figure 2. Shear strengthening with externally bonded CFRP laminates

The externally bonded CFRP shear reinforcements have to be applied over the complete height of the side of the element. If more than two layers are required, the horizontal and vertical layers have to alternate. The influence of externally bonded CFRP shear reinforcement on the behaviour of concrete beams has been studied extensively in Ahmed4.

STRENGTHENING OF CONCRETE PLATES

In general, the force distribution in plates is determined by the ratio of the stiffnesses and the ratio of the spans in both directions. When the stiffness in both directions is equal, the plate is called “isotropic”. A reinforced concrete plate can be considered as isotropic, since the amount of internal steel in both directions generally only slightly differs. The distribution of the bending forces in the plate then only depends on the ratio of the spans. The biggest part of the load will be taken by the shortest span.

Once the force distribution in each direction is determined, the calculation of the amount of internal or external reinforcement is identical to the design of reinforced concrete beams subjected to bending, see Van Gemert5 . When externally bonded steel plates are used for strengthening or stiffening reinforced concrete plates, they generally can only be applied in one direction, which is preferably the shortest span. By doing so, the amount of steel area in the direction of the external steel plates will strongly increase, and consequently the stiffness in that direction will strongly differ from the stiffness in the transverse direction. This orthotropy in the stiffness characteristics has a great influence on the distribution of forces in the plate. The stiffer the plate in one direction, the greater the relative part of the load which will be taken by that direction and also the greater the moments will be in that direction. The new load distribution is given by the orthotropic plate theory . In the case of an increase of the allowable load on a reinforced concrete plate, the unstrengthened plate usually will have insufficient steel reinforcement in both directions. When one direction is strengthened with steel plates, the increased bending moments in that direction have to be carried by both the internal and external steel area. Moreover the bending moment in the transverse direction have to decrease so that it can be carried by the internal steel reinforcement in that direction. This means that one direction has to be overstrengthened in order to unload the other direction. The advantage of the thin CFRP laminates is the fact that they can be applied in both directions. In that way, the strengthened concrete plate remains isotropic, which means that the relative force distribution will not change. Since reinforced concrete plates are much thinner than concrete beams, the lever arm from the resulting concrete compressive force to the external reinforcement is much larger than the lever arm to the internal reinforcement. This means that the active tensile stresses in the external reinforcement can be much higher than in the internal steel reinforcement, which is very favourable for the high strength CFRP laminates. Therefore CFRP laminates can often be used more effectively for strengthening concrete plates than for strengthening concrete beams. However, when CFRP laminates are used, the deformations might become unacceptable. Since the area of the externally bonded CFRP laminates is rather small, the bending stiffness of the concrete plate will only slightly increase, possibly causing important deflections. Therefore it is necessary to check also the deformations of a strengthened reinforced concrete plate. For prestressed concrete plates, strengthening with externally bonded reinforcement is mostly not possible. Since the prestressing already causes considerably high concrete compressive stresses - often close to the concrete compressive strength - the bearing capacity can not be increased considerably by bonding external reinforcements.

ANCHORAGE DESIGN

A good understanding of the stress distribution at the end zones of the externally bonded reinforcement is essential for the proper design of the plate and laminate end anchorage. Premature failure often occurs in these zones. The force transfer between the concrete member and the external reinforcement takes place by means of shear stresses. The knowledge of this shear stress distribution is essential for the design of the end anchorage and the determination of the bearing capacity of the bonded connection. For the practical design, two values have to be determined: the maximum transferable force and the anchorage length.

The maximum transferable force is the maximum force that can be transferred by a bonded connection in shear and the anchorage length is the length needed to transfer that maximum transferable force. The derived formulae are based on the theory of non linear fracture mechanics, see Brosens6 . A bilinear shear stress-slip relationship for concrete is used and both pre- and post-cracking behaviour is taken into account. According the model a simple formula can be obtained to calculate the anchoring length. This anchoring length has to be provided at both ends of the external reinforcement. If there is no place to add this supplementary length or if the maximum transferable force of the connection is insufficient to carry the acting forces, an additional mechanical anchorage system is necessary. For steel plates, this mechanical anchorage can be realised by providing a bolt, whereas for CFRP laminates only an external stirrup is possible, figure 3.

CFRP plate Steel plate

CFRP stirrup

Steel bolt

Figure 3. End anchorage of externally bonded reinforcement

CASE STUDY: ROOF STRUCTURE WHAREHOUSE In 2002, the supporting beams of a roof structure of a stockroom at Brussels in Belgium were strengthened using a hybrid strengthening technique, see Brosens et al7 . Due to a calculation error, the bearing capacity of the main supporting beams was only half as much as needed. The amount of internal steel tensile reinforcement was unsatisfactory. The beams have a span of 14m and a rectangular cro ss section of 400mm x 800mm. The roof structure consists of two adjacent spans of these simply supported beams. The concrete beams were prefabricated and already positioned on site. To prevent a delay of the construction works, the option of strengthening was chosen instead of replacing the beams. The first idea was to provide additional steel plates at the bottom side of the beams. To attain the desired bearing capacity, very long steel plates (8m) with a relatively large steel cross section (200mm x 20mm) would be needed. To avoid such heavy steel plates, which are very difficult to apply and to anchor to the concrete beam, another strengthening concept was chosen. Above the mid support, the two beams were connected to each other in order to obtain a hyperstatic structure. In that way, the bending moments at midspan of the beams can be reduced and consequently also the amount of additional external reinforcement in the midspans.

The quality of the concrete was C30/37 and the steel bar grade was BE500. The bending capacity of the original beams was 460.6 kNm which is sufficient to carry the own weight of the structure (concrete beams and steel deck) but insufficient to carry an eventual maintenance loading of 75 kg/m2 . The design of the additional reinforcement was made, assuming that the dead weight of the structure is taken isostatically by the beams, whereas the maintenance loading is taken hyperstatically. This leads to an additional bending moment of 475.6 kNm at the midspan and of 120.8 kNm at the support. To obtain the required bearing capacity and stiffness of the structure, a steel plate with a cross section of 150mm x 8mm is applied at the bottom side in the midspan of the beams . To prevent premature delamination failure and plate end shear failure, the steel plate is anchored into the concrete beams by applying 3 bolts M16 at each end of the plate. At the mid support, the two beams are connected by providing a steel plate (length 7m, cross section 200mm x 8mm) at the top side of the beams. This steel plate will transfer the tensile stresses, originating from the negative (hyperstatic) bending moment at the support. Both ends of the steel plate are anchored to the concrete beam with 4 bolts M16. To enable the transfer of compressive stresses, the opening between the two beams is filled with an epoxy repair mortar. To prevent additional cracking due to thermal forces in the lower part of the beam above the support, additional tensile reinforcement is added in this area. Flexible CFRP sheets were used for that purpose. At each side of the beam, 3 layers of CFRP (length 4m, equivalent fibre thickness 0.167mm/layer, width 200mm) were applied. Finally two stirrups made of CFRP sheets (2 layers, equivalent fibre thickness 0.167mm/layer, width 300mm) were provided to take up the increased shear forces due to the connection of the two beams,. Figure 4 gives an overview of the hybrid strengthening of the supporting beams of the roof structure. The complete strengthening work took only 2.5 weeks, which obviously is much faster than the alternative of replacing the beams by new ones. Some details of the application of the externally bonded reinforcement are shown in figures 5 and 6.

Figure 4. Overview of the hybrid strengthening work

Figure 5. Externally bonded steel plate at the bottom side of the beam

Figure 6. Detail of the connection of the two beams

CONCLUSIONS

A good understanding of the force distribution in externally bonded reinforcement is essential for the design and the application of the technique in practice. The choice of the strengthening material, steel plates or CFRP laminates, is very important. Steel plates are suitable for the enhancement of both strength and stiffness properties whereas CFRP laminates are mostly only suitable for the enhancement of the strength properties. For the design of the cross sectional area, bending as well as shear forces have to be considered. It is very important to take into account the original stress situation before applying the external reinforcement. Only in that case an appropriate design is possible. According to the eurocode standards, both the ultimate limit state and the serviceability limit state have to be checked. An important issue is the design of the end anchorage. Since the plate end is a discontinuity, high shear stress concentrations might cause premature peeling failure in the end zones of the external reinforcement. Therefore, the maximum transferable force and the anchorage length have to be determined. If the force capacity of the connection is insufficient, an additional mechanical anchorage, such as bolts or external stirrups, has to be provided.

REFERENCES

1. Brosens K., Ahmed O, Van Gemert D. and Ignoul S., ‘Strengthening of R.C. Beams - Hybrid steel/CFRP solutions’, Structural faults & Repair 99, 8th international conference, 13-15 July 1999, London, England

2. Van Gemert D., Vanden Bosch M. and Ladang C., ‘Design method for strengthening reinforced concrete beams and plates’, 2nd edition, 32-ST-17, K.U.Leuven, Belgium, 1990

3. Eurocode 2, NBN B15-002 - ENV 1992-1-1: ‘Design of concrete structures - Part 1-1: General rules and rules for buildings’, 1st edition, December 1995, BIN

4. Ahmed O., ‘Strengthening of R.C. beams by means of externally bonded CFRP laminates - Improved model for plate end shear’, Doctoral thesis, K.U.Leuven, 2000

5. Van Gemert D., ‘Special design aspects of adhesive bonding plates’, ACI SP165, Repair and strengthening of concrete members with adhesive bonded plates, Ed. Swamy N. and Gaul R., 1996, pp. 25-42

6. Brosens K., ‘Anchorage of externally bonded steel plates and CFRP laminates for the strengthening of concrete structures’, Doctoral Thesis, K.U.Leuven, May 2001

7. Brosens K. and Van Gemert D. (2001), Internal report 241, Triconsult N.V., 2001